The Briton pulled out of the world championship race after some 140 kilometres, having worked on his own at the front of the peloton for 100 kms knowing he had no chance of retaining his title.
"I'm more tired after 140k than after 250k last year at the world championships," Cavendish said.
Cavendish had worked tirelessly for Team Sky leader Bradley Wiggins during the Tour de France and he did not hesitate when asked to be a domestique for British team mates Jonathan Tiernan Locke and Steve Cummings on Sunday.
Cavendish put on a happy face on his last day as world champion.
"With one minute to go (before the start), I was like: 'I've got one minute left as world champion'," he said.
"As a world champion it's been a good year. The last more successful person was Tom Boonen in 2006 when he won (the world title) in 2005; 15 wins is not bad with the world champion jersey."
Having ridden the Tour as a domestique, sacrificing his chances of claiming a second green jersey for the points classification, Cavendish could be tempted to change teams.
Although he is still contracted to Team Sky, he has been widely reported to be about to join Belgian outfit Omega Pharma-Quick Step.
Asked when he would clarify his situation, Cavendish said: "I don't know. It's not up to me."